How to rotate a 2d plan in ver 2.79. no option in propert panel
Assuming you have a blueprint which contains sections with both plan and elevation views, you can assign the same image as background to each relevant Blender orthographic view. 

For example: Having constructed your shell from the blueprint's elevation section in the Blender's 'Left' view, you can add (Add Image) and assign the same blueprint image to Blender's 'Top' view. You don't have to reload it from file, it will already be in Blender's list of available images. You then offset/ rotate/reflect the blueprint image (in the Blender's background image panel) so that its plan section coincides with the shell you have built, seen from the top. 

The versions of the background image are visible only from the exact preset Blender view to which they were assigned.